In a deep drilling system, a drill bit is typically connected via several drill pipes, forming a drill string, to a drilling motor. Such a setup is also generally illustrated in FIG. 1. The drilling motor, provided on the earth's surface, applies drilling forces—the longitudinal and rotational forces as illustrated by arrows in FIG. 1—onto the drill string 1 such that the drill bit 3 advances further into the ground 4, thereby creating a bore hole 5. Since bore holes can reach depths of up to several kilometers, it is desired that the drill string 1 is centered in the bore hole. Particularly, the sections close to the drill bit are centered such that the drill bit 3 advances in a defined direction into the ground. For these reasons, stabilizers are typically utilized, which can be provided in form of blades 2 as illustrated in FIG. 1. These blades are fixed to the drill string 1 and extend to the walls of the bore hole 5.
During drilling operation, a water-based drilling fluid is commonly pumped downwards through the drill pipes to the drill bit, such that it flows back in the space provided between the drill string 1 and the walls of the bore hole. Thereby the drill bit is cooled, and the cuttings are transported to the surface.
When advancing through certain materials, e.g. when advancing through shale formation, the shale reacts with the water, swells and becomes sticky. This sticky mud, or sticky cuttings, can adhere between the blades 2, forming a ball of mud or mud cake 7. This effect, exemplarily illustrated in FIG. 2, is known as “balling”. Thereby a cavity 6 of the bore hole 5 can be formed wherein the diameter of the mud cake 7 can become bigger than the inner diameter of the bore hole 5.
This balling can create problems, in particular during pulling-out-of-hole (POOH) and/or running-in-hole (RIH) operations. For example, as illustrated in FIG. 3, when trying to retrieve the drill string out of the hole (POOH), problems arise: The accumulated mud cake 7 can cause a severe drag, and even jam the movement of the drill string. Accordingly, POOH operations can take much longer due to balling, or in the worst case, the drill string cannot be removed from the hole at all and has to be cut off.
Typically such balling is characterized by an increased necessary rotary torque and a reduced penetration rate during drilling. Accordingly, balling can be noticed by an operator. Several methods for unballing are known in the art. For example, when balling has been noted, the drill bit can be lifted off the bottom of the bore hole and the water flow rate can be increased for a certain amount of time. Further, by spinning the drill string as fast as possible, it can then be tried to fling off the mud cake. Alternatively, it can also be tried to shake off the mud cake by lifting and dropping the drill string rapidly. It can also be tried to pump a relatively small volume of specially prepared fluid—a so called “pill”—placed or circulated in the bore hole and subsequently wash off the ball of mud. By pumping fibers in the drilling fluid, it is intended to provide a better hole cleaning. Other techniques for preventing balling are based on providing a special coating onto the drilling equipment. However, these techniques are either expensive in cost, suited for one-time use only, or are ineffective in solving the issue.
The document EP 0 285 889 B1 discloses a deep drilling tool having displacable stabiliziers, wherein the stabilizers are displaced by means of variation of the drilling fluid pressure.
It is therefore an object of the present invention to provide a system which better deals with balling than the prior art techniques or systems such that POOH and/or RIH operations can be performed easier and faster.
These and other objects, which become apparent by reading the following description, are achieved by the present invention according to the subject matter of the independent claims.